Reducing level shifter standby power consumption

ABSTRACT

A level shifter may be utilized in a flash memory to reduce the standby power consumption. The level shifter may be coupled so that the gate-to-source voltage of the input transistor is reduced during standby operations to reduce leakage current. At the same time, the source of the input transistor may be coupled to a lower voltage during active level shifting operations. Thus, good transistor characteristics may be achieved with reduced leakage currents.

BACKGROUND

This invention relates generally to level shifters that are utilized for example in shifting voltage levels in memories such as flash memories.

Flash memories may need a plurality of different voltage levels to perform read, programming and erase functions. Generally these voltages are produced by on chip charge pumps. A charge pump is a DC-to-DC voltage converter. The charge pump generates a desired voltage level by selectively clocking capacitors and transistors in a prescribed sequence.

Charge pumps may include a number of different stages. The amount of input current to the charge pump is generally proportional to the number of those stages. The voltages generated by the charge pump are eventually coupled to the gate, drain and source terminals of flash memory cells in a flash memory array by way of decoder and switch circuits. The decoder and switch circuits may be constructed using complementary metal oxide semiconductor (CMOS) transistors. As a result, they may have a predetermined amount of associated capacitance.

The capacitance of the decoder and switch circuits may be significant. As a result, it may be necessary to keep the decoder and switch circuits in a ready state for memory read operations to achieve fast read access times. Thus, this capacitance is normally charged to prescribed read voltage levels during read mode. The read mode may include sequences when memory reads occur and intervening time periods between memory operations. During the intervening periods, the chip may assume a low power or standby state. Thus, this charging of capacitance, even in the low power or standby state may result in substantial leakage.

Subthreshold leakage occurs in these decoder and switch circuits. The leakage is significant because the decoder and switch capacitance must be fully charged even during standby. The leakage results in input current and therefore adds to the average current consumed during standby.

The widespread use of portable systems with on-board flash memories increases the need to limit power consumption. Even relatively small amounts of standby power consumption in devices with a large number of transistors may result in considerable drain in battery powered systems. Limited battery life means that reducing overall power consumption is a critical design goal.

Leakage power consumption occurs in the absence of a switching event. Leakage power consumption may be due to non-zero reverse bias leakage and subthreshold current. With a large number of transistors, leakage power consumption can significantly contribute to overall power consumption. Subthreshold leakage is due to carrier diffusion between the source and drain regions of transistors in weak inversion. An MOS transistor in the subthreshold operating region may behave like a bipolar device, exhibiting exponential dependence on the gate-to-source voltage. Particularly when the gate-to-source voltage is smaller than, but very close to, the threshold voltage of the device, subthreshold leakage current may become significant.

Level shifters may be used in connection with the decoder and switch circuits. Steering logic may be utilized to select the appropriate voltage for a given memory function such as read, program or erase. Level shifters may experience significant standby power consumption.

Thus, there is a need to reduce level shifter standby power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block depiction of a flash memory in accordance with one embodiment of the present invention;

FIG. 2 is a circuit diagram for a level shifter in accordance with the prior art; and

FIG. 3 is a circuit diagram for a level shifter in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, a flash memory 10 may receive a supply voltage that may be boosted using one or more boosters or charge pumps 12 to one or more voltages indicated as V_(ext). For example the boosted voltage V_(exta) may be a read voltage (e.g. 5 v.) and the boosted voltage V_(extb) may be the program voltage (e.g. 11.5 v.). Each boosted voltage (V_(ext)) may then provide a supply voltage to a switch 13 including P-channel transistors 15. The external voltage V_(ext) may than be supplied to a plurality of level shifters 14 that turn the P-channel transistors 15 on and off via the level shifter 14 output voltage (V_(out)). The control logic 11 supplies an input voltage (V_(int)) to each level shifter 15.

The output voltage from the switch 13 may be applied to an address decoder 16. The address decoder 16 may receive off chip address information 20, buffered by the address buffer 18. The address decoder 16 then supplies the appropriate voltage level signals to memory cells in the memory array 22.

Referring to FIG. 2, a level shifter 14 a, in accordance with the prior art, may experience significant standby subthreshold leakage current. The level shifter 14 a receives the input voltage V_(int) via the gate of an N-channel MOS input transistor 28. The input transistor 28 has its drain coupled to an output voltage (V_(out2)) node and its source coupled to ground. The input voltage V_(int) may also be coupled through an inverter 32 to another N-channel MOS transistor 30 that also has its source grounded. The drain of the N-channel transistor 30 may be coupled to a output voltage (V_(out1)) node.

The drain of each N-channel transistor 28 or 30 is coupled to P-channel cross-coupled transistors 24 and 26. The sources of the P-channel transistors 24 and 26 may be coupled to an external voltage V_(ext).

In operation, when the input voltage V_(int) is low (such as zero volts), the input transistor 28 does not turn on. As a result, the output voltage V_(out2) may rise to the level of the voltage V_(ext). This is because the drain of the input transistor 28 is at a high potential, turning off the transistor 26. At the same time, the N-channel transistor 30 receives a high, inverted signal on its gate from the inverter 32. This gate voltage enables the transistor 30 to couple the V_(out1) node to a ground or low potential. As a result, V_(out1) goes to ground and V_(out2) goes to the voltage V_(ext).

Even with a low level output voltage V_(int), significant subthreshold leakage may be experienced by the N-channel transistor 28. Subthreshold leakage current is an exponential function of V_(gs), the gate-to-source voltage, minus the threshold voltage (V_(t)) of the input transistor 28. When the input transistor 28 has a zero gate voltage and a grounded source bias, V_(gs) is effectively zero volts. Thus, the subthreshold leakage is as follows:

I _(sub) =I ₀(^((Vgs−ΔVt)/s)−1)

where I₀ is the leakage when V_(gs)−V_(t)=0 and s is the subthreshold slope which is the rate of change of current through the transistor as a function of V_(gs). ΔV_(t) is V_(to)−V_(tbs) where V_(to) is the threshold voltage when V_(bs) is equal to zero and V_(tbs) is the threshold voltage when V_(bs) is not equal to zero.

Thus, when V_(gs)−ΔV_(t) is negative, the subthreshold leakage current may be equal to the subthreshold leakage current at V_(gs)−ΔV_(t)=0 times a significantly small value. This may be a substantial reduction in the amount of leakage current.

During standby conditions, input transistor 28 operating or performance characteristics may not be very critical. There is no need to have a relatively high gate-to-source voltage in order to have effective input transistor 28 performance. Thus, during standby, the gate-to-source voltage of the input transistor 28 may be set relatively low. Conversely, during level shifting operations, a more substantial gate-to-source voltage may be advantageous.

Referring to FIG. 3, a level shifter 14 may be similar to the level shifter 14 a in FIG. 2. The source of the N-channel input transistor 28 may be coupled to the output of the inverter 32. Thus, in a standby mode, the source of the input transistor 28 is at a relatively higher voltage. During normal level shifting operations, the source of the input transistor 28 may be at a relatively lower voltage. As a result, during standby operations, the voltage V_(gs) of the transistor 28 may be a larger negative voltage, related to a supply voltage. Conversely, during regular mode, when level shifting is occurring, V_(gs) may be a positive voltage to provide desirable transistor operating characteristics.

As a result, the leakage current, in standby, may be reduced as much as one hundred times or more. This may result in current savings of a microamp or more depending on memory architecture. Coupling the source of transistor 28 to a positive voltage not only makes V_(gs) negative, it also increases the threshold voltage of the transistor 28, further reducing subthreshold current.

In some embodiments, the subthreshold leakage current may be reduced for positive current pumps, allowing for lower standby current. Also, lower supply voltages may be used to achieve standby currents close to those that are offered on similar chips with higher supply voltages. Since the leakage current is reduced, faster flash memory read access times may be achieved, in some embodiments, because the internal capacitance may be charged to the read voltage while still meeting the desired standby current consumption requirements.

More than two voltages (read and program) may be handled in other embodiments of the present invention. Also, a different inverter may be used to drive the source high. The same approach may also be and with the transistor 30.

While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention. 

What is claimed is:
 1. A level shifter comprising: an input transistor having a gate-to-source voltage; a device to reduce the gate-to-source voltage of the input transistor during standby mode and to increase the gate-to-source voltage of the input transistor during operating mode.
 2. The level shifter of claim 1 wherein said device reduces the subthreshold leakage current during standby mode.
 3. The level shifter of claim 2 wherein said device provides a higher source voltage to said input transistor during standby mode.
 4. The level shifter of claim 3 wherein said device provides a lower source voltage to said input transistor during operating mode.
 5. The level shifter of claim 4 wherein said device includes an inverter.
 6. The level shifter of claim 5 wherein said input transistor is an N-channel input transistor.
 7. The level shifter of claim 6 wherein the source of said input transistor is coupled to the output of said inverter.
 8. The level shifter of claim 1 wherein said device increases the threshold voltage of the input transistor in the standby mode.
 9. The level shifter of claim 8 wherein said device increases the threshold voltage of the input transistor causing the gate-to-source voltage to become negative.
 10. The level shifter of claim 1 wherein said input transistor has a gate, source and drain, the gate coupled to receive an input to the level shifter, said device including an inverter coupled to the gate, the inverter having an input and an output, the source of the input transistor coupled to the inverter output.
 11. The level shifter of claim 10 including a pair of P-channel transistors, each including a drain, one of said P-channel transistors having its drain coupled to the drain of the input transistor.
 12. The level shifter of claim 11 including a second N-channel transistor, said second N-channel transistor coupled to the drain of one of said P-channel transistors, said second N-channel transistor having a gate, source and drain.
 13. The level shifter of claim 12, wherein the output of the inverter is coupled with the gate of said second N-channel transistor.
 14. The level shifter of claim 13, wherein the source of said second transistor is coupled to the ground.
 15. A method comprising: reducing the gate to source voltage of an input transistor of the level shifter during standby mode; and increasing the gate to source voltage of the input transistor of a level shifter during operating mode.
 16. The method of claim 15 including reducing the subthreshold leakage current during standby mode.
 17. The method of claim 16 including providing a higher source voltage to said input transistor during standby mode.
 18. The method of claim 17 including providing a lower source voltage to said input transistor during operating mode.
 19. The method of claim 18 including inverting the input signal to said input transistor.
 20. The method of claim 19 including applying said inverted signal to the source of said input transistor.
 21. The method of claim 20 including applying an output voltage of said level shifter to the switch of a flash memory.
 22. The method of claim 21 including receiving an external voltage from a charge pump.
 23. The method of claim 21 including increasing the threshold voltage of the input transistor in the standby mode.
 24. The method of claim 23 wherein increasing the threshold voltage includes causing the gate to source voltage to become negative. 